Films of dielectric materials have been observed to be capable of acquiring a static electric charge upon being poled (i.e., being exposed to a dc field) which static charge is quasi-permanent, the degree of permanency being dependent largely upon the chemical constitution of the film. Such a charge, due largely to trapped positive and negative carriers positioned at or near the two surfaces of the dielectric respectively, produces what is known as an electret.
In addition to being capable of acquiring a static electric charge (and, as a result acquiring an electrostatic field), a certain limited class of film also acquires an ordered internal molecular orientation when exposed to a dc field so that even when the static (or electret characterizing) charges are dissipated, the film thereafter possesses the property of being able to generate an electric current between the electrically connected opposed surfaces by changing the pressure imposed on the opposed surfaces. This property is known as the piezoelectric effect and some such film (e.g., film made from polyvinylidene fluoride (PVDF) also possesses a pyroelectric effect (i.e., the property of producing electron flow by changing the temperature of the film). Also, when a voltage is applied to the opposite surfaces of a piezoelectric film, a reverse piezoelectric effect occurs, i.e., the film deforms physically in proportion to the voltage applied to it.
The production of electrets and piezoelectric film by poling is well known in the art. A preferred system for the continuous poling of piezoelectric sensitive film using a corona discharge to induce the piezoelectric charge is described and claimed in U.S. applications to Radice, Ser. Nos. 197,463 and 197,466, each filed Oct. 16, 1980, now U.S. Pat. Nos. 4,392,178 and 4,365,283, and incorporated herein by reference. Regardless of the mechanics of poling, it has been observed that poled film possesses unstable charges, sometimes described as unstable homo- or hetero-charges. Until such unstable charges are removed, the piezoelectric or pyroelectric properties of the poled structure will vary. The art teaches several techniques for removing such unstable charges as by use of pressure while the two surfaces of the poled structure are short circuited (U.S. Pat. No. 4,055,878, dated Nov. 1, 1977), by use of heat treatment (U.S. Pat. No. 3,660,736, dated May 2, 1972 and U.S. Pat. No. 3,833,503, dated Sept. 3, 1974) and by immersion in water (U.S. Pat. No. 3,793,715, dated Feb. 26, 1974). The present invention provides a quicker and more efficient way of removing such unwanted charges.